By way of background, anemometers measure the flow of fluids and take many forms. For macro measurements such as wind velocity, wind turbine and wind vanes are common. For measurements requiring a small sensor for profiling and other applications, a popular device is the hot wire or hot film anemometer. This configuration exploits the resistive temperature coefficient of materials by fabricating them into thin film or wire sensors. Additionally, these sensors may be oriented 90 degrees to each other to provide three orthogonal components of the fluid velocity vector. In practice, the sensor is heated to a temperature above ambient. As the sensing element is cooled by fluid flow, energy is carried away, lowering the temperature and causing a change in resistance. This change in resistance is quite small and therefore significant amplification is required to develop a useful signal for measurement.
There are three common methods of heating the wire and measuring its resistance: the constant voltage, the constant current, and the constant temperature method. All three of these methods use a feedback control system to maintain a parameter that is kept constant. All methods involve sensing the voltage across the heated element and use the sensed voltage for controlling the loop or making measurements. However, existing constant temperature hot wire/film anemometer designs are plagued by three particular issues, namely:                1. The need to adjust system damping—which the operator must perform for each channel and with every change of sensor or cable.        2. The process for adjusting the system to achieve proper sensor drive is cumbersome and can itself be destructive of the hot wire/film sensor element.        3. The frequent destruction of the delicate and expensive sensors from endemic current surges.        
Each of these problems will be discussed in detail herein below.
Issue #1, adjustment of system damping is an elaborate and essentially unnecessary process to minimize, but not eliminate, system instabilities. The hot wire or film to be sensed is part of a probe used for measuring fluid flow at a specific location and orientation. Because the point of sensing is remote from the point of data acquisition, connections between sensors and signal processing hardware exists, facilitated with cabling.
Some form of shielded wire is used to prevent noise pickup in the interconnection between sensors and signal processing electronics. Coaxial cable and twisted pairs are used at audio frequencies in many home and commercial installations. At higher frequencies, shielded wires exhibit transmission line characteristics such as the reflection of energy if the wires are not properly fed and terminated.
The bandwidth of many hot wire/film anemometers is similar to the audio frequency range of 20 Hz to 20 kHz and transmission line effects are not considered. However, the control loops that are used to create the constant current, voltage, or temperature have bandwidths well exceeding the audio frequency range. The wires between the sensor and circuits therefore constitute a transmission line and must be treated as such. An improperly fed or terminated transmission line is capable of exhibiting time delay effects and can present a complex impedance; i.e. having a reactive component. The confluence of sensor, cable, and circuit can then satisfy the Barkhausen Criteria, pushing the circuitry into oscillation. This is the reason for the complicated process to adjust the damping, partially stabilizing the system. However, if the sensor or cable is exchanged, the frequency of oscillation shifts, and the tuning process must be repeated.
A control loop with a complex impedance present at any point is subject to instabilities. While hot wire or film anemometer systems have employed additional components and lengthy procedures to mitigate ringing of an underdamped system as well as other effects, these attempted solutions have not eliminated such problems.
Issue #2, the cumbersome process for setting the sensor drive, exists because of the need to adjust the drive based upon the cold resistance of the individual sensor. Typically, the drive resistance is set to approximately 1.8 times the cold resistance of the sensor. When the unit is energized, the bridge excitation increases to heat the sensor to raise its resistance to match the drive resistance. Existing designs require measuring the sensor cold resistance, performing the necessary mathematics, and then adjusting the drive setting resistance to the necessary value to achieve the desired drive. This is typically done either by disconnecting and reconnecting components or, by using expensive resistor switching units with numerical displays that must be set accordingly.
Issue #3, the too-frequent destruction of the sensor element from circuit transients, results from the propensity of existing hot-wire anemometer designs to produce and transmit large and fast current spikes to the connected sensor. This occurs when the sensor parameters are changed too quickly or functions are performed ‘out of sequence’.
The present invention provides an anemometer design that encompasses three new circuit topologies addressing these and other problems in the art.
Those skilled in the art will appreciate the above stated advantages and other advantages and benefits of various additional embodiments upon reading the following detailed description of the embodiments with reference to the below-listed drawing figure.
According to common practice, the various features of the drawing discussed below are not necessarily drawn to scale. Dimensions of various features and elements in the drawings may be expanded or reduced to more clearly illustrate the embodiments of the disclosure.